1. Field of the Invention
This invention relates to polyimide resins which, because of their high thermal and oxidative stability and high strength at elevated temperatures, are useful in high temperature applications, including jet aircraft components, such as blades, seals, air splitters, and engine casing parts.
2. Description of Prior Art
Polyimide resins are generally useful in applications requiring both high strength and high temperature capabilities, as where temperatures may reach up to approximately 550.degree. F. (300.degree. C.). Nevertheless, use of these polyimide resins has been limited by economic factors. In particular, a high energy expenditure is required to cure the polymer composition, and the preparation can be time consuming.
Addition cross-linking of high temperature polyimide polymers has, in the past, employed either norbornenyl (nadic) end-capped imide monomers or oligomers or maleic endcapped monomers or oligomers.
Norbornenyl-capped systems are typically cured at temperatures near 300.degree. C. to give polymers able to function as matrix resins at use temperatures near 300.degree. C. Attempts to modify the chemical reactions to reduce the cure temperature have succeeded in doing so, but only at the cost of lowering the ultimate use temperature of the final polymer. Thus, for example, maleic-capped systems exhibit more desirable cure temperatures (on the order of 200.degree. to 275.degree. C.). However, the resulting polymers cannot be used as matrix resins as sustained temperatures of 300.degree. C.
Furthermore, thermally initiated copolymers of maleic anhydride (and derivatives thereof, such as maleimide) with a wide range of vinyl compounds (such as styrene, acrylonitrile, and methylmethacrylate) are known.
U.S. Pat. No. 3,745,149 to Serafini, et al., the disclosure of which is incorporated by reference herein, discloses a method for the preparation of polyimides from mixtures of monomeric diamines and esters of carboxylic acids. These starting materials may also be used in the practice of the present invention. As taught by this patent, macro-molecular polyimides are prepared from mixtures of monomers of (a) a dialkyl or tetra-alkyl ester of an aromatic tetracarboxylic acid, (b) an aromatic diamine, and (c) a monoalkyl or dialkyl ester of a dicarboxylic acid, wherein the ratio of (a):(b):(c) is N:(N+1):2, wherein N is an integer of from 1-20. These monomers are mixed in an organic solvent and comprise 30-70 percent by weight of the solution. The resulting solution of monomers has excellent long-term shelf life.
These monomers are nonreactive at room temperature, but react at elevated temperatures to form prepolymers, and when cured at high temperatures, form macromolecular polyimides having molecular weights of at least 10,000.
These polyimides have excellent high temperature oxidative and thermal stability and outstanding mechanical properties. However, their uses are limited by economic factors. In particular, the high energy input and time expenditure associated with the application of heat to raise the temperature to near 300.degree. C. to effect the curing of the polymer, limits the use of the polyimides of this invention to very select applications.
U.S. Pat. No. 3,179,634 to Edwards and U.S. Pat. No. 3,565,549 to Lubowitz et al., also deal with end-capped prepolymers similar to those of the present invention.